What's the purpose of Node Wrangler's Viewer Node?
It's a technical detail but using the Emission Shader as some kind of bridge is technically correct.



The color of each socket represents a specific data type. Quoting from What is the meaning of the color of the node sockets in the node editor?:


  
  Grey is a single value
  Yellow is a tuple containing 3 values; the red, green, and blue channels of a color.
  Purple is a tuple containing 3 values. Used for vector/coordinate information.
  Green is a shader closure, representing a description of how light will interact with a surface or volume. This one can only be connected into other green sockets.
  


As you can see the Surface socket from the Output node is green so it expects an incoming shader (closure) and no other kind of data. However, that you're allowed to plug any other node type into the Surface Socket is actually a great feature...



For the sake of completeness: In case a float socket (grey) is connected to rgb (yellow) the resulting g and b channels are just a copy of r to fill it up.